A. Field of the Invention
The invention relates to ionic separation techniques such as capillary ion electrophoresis and ion chromatography and, more particularly, to detectors for such techniques, especially conductivity detectors.
B. Prior Art
Capillary ion electrophoresis is a well known and highly useful technique for detecting and measuring ionic solutes. In this analytic technique, an analyte is flowed through a small bore capillary tube, along which an electric field is established. Typically, the field is on the order of 300 volts/meter. Ion constituents in the analyte move along the field at a rate dependent on their mobilities in the analyte solute. At the end of some period of time (e.g., several minutes in the case of a capillary tube on the order of a meter in length), the analyte zones at the discharge end of the capillary becomes highly concentrated with respect to the various ionic solutes within the analyte. This concentration changes as a function of time, characteristic of the different mobilities of the ions in the analyte solution.
A variety of means are used to detect the concentration of the solutes at or near the discharge end of the capillary. These include optical techniques such as fluorescence and ultraviolet absorption measurements; spectrometry; radioisotope tagging; and electrochemical and conductive measurements on the analyte, among others. The detectors may be placed "on-column" (that is, at a point near, but prior to, the discharge end of the capillary) or "off-column" (that is, post discharge).
Conductivity measurement is a form of detection that has been found to be especially simple for the user, reliable, accurate, and universal; thus it has become highly desirable for use in ion electrophoresis analysis. Both on-column and off-column conductivity detectors have been used. Off column detectors are less susceptible to the interference that commonly arises from the high voltage along the column required for ion separation, and have a lower cell constant and thus higher sensitivity. However, they frequently suffer from diminished resolution. A brief review of such detectors, and an analysis of one form of off-column detector, is given in Huang and Zare, End Column Detection For Capillary Zone Electrophoresis, Anal Chem. 1991, 63, 189-192.